Conventionally, a drive and brake device for a bicycle has been proposed which is provided with a coaster brake mechanism housed within a rear wheel hub, a plurality of driving rear gears provided on a brake actuator for actuating the coaster brake mechanism, and a derailleur having two pulleys and being provided in the vicinity of the driving gears, so that a drive chain is stretched across the front gear and one of the driving rear gears and switched to a desired one of the rear gears through the derailleur. The front gear is normally rotated to drive the rear hub at a given speed change ratio, and reversely rotates to allow the rear gears to reversely rotate to thereby actuate the brake actuator for exerting the braking action.
In the above device, a tension spring is provided at the derailleur to keep the chain always in constant tension even when it is switched to the rear gear of a different diameter. However, when the front gear is reversely rotated for exerting the braking action, the chain, between the pulley and the front gear, is tensioned on the tight side or drive side, whereby a changeover cage carrying the pulleys swings to the utmost extent against the tension spring, and thereafter the rear gear reversely rotates.
Accordingly, during the swinging motion of the changeover cage, no rear gear reversely rotates even with reverse rotation of the front gear, thereby creating a delay of the braking effect. The utmost swinging motion of the cage applies an excessive force to the derailleur to lead to a breakdown therein. Also, since the respective rear gears are reversely rotated for exerting the braking action, a rotary angle of the front gear, in other words, an angle of revolution of a crank arm, differs due to different gear ratios, thereby making it difficult to exert a stable braking action.
Furthermore, the utmost swinging motion of the changeover cage during the braking action causes a slack in the chain on the drive side between the rear gear and the front gear during the normal rotation thereof, whereby the front gear, in transition from the braking condition to driving condition, is in lost motion to the extent of the chain's slack, resulting in the chain disengaging from the front gear.
Conventionally, a device has been proposed which is provided with a front braking gear at a crank shaft carrying the front gear, a rear braking gear mounted on the rear hub, and a braking chain stretched across the front and rear braking gears, so that the front braking gear is rotated together with the front gear by back-pedalling so as to rotate the rear braking gear through the braking chain, thus exerting the braking action. Such a device, however, additionally requires the braking gears and braking chain, resulting in a complicated construction which is expensive to produce.